Recently, short-wavelength semiconductor lasers have been developed so that these lasers may be applied to high-density optical disk systems, etc. In order to increase the recording density of recording media, there has been a demand for lasers with shorter oscillation wavelengths. As a short-wavelength semiconductor laser, a 600 nm band light source formed of InGaAlP material has already been put to practical use because the characteristics thereof are improved enough to perform information write/read.
In order to further increase the recording density, blue semiconductor lasers have been intensively developed. Oscillation operations of semiconductor lasers using Groups II-VI elements have already been confirmed. However, it is still difficult to put these lasers to practical use, since the term of reliability is limited to about 100 hours and oscillation is difficult at wavelengths of 480 nm or less. In order to apply these lasers to the next-generation optical disk systems, etc., there are many limitations to semiconductor materials.
On the other hand, with GaN semiconductor lasers, the wavelengths can be decreased to 350 nm or less. In the case of LEDs, it has been confirmed that if proper conditions are chosen, the term of reliability can be increased to 10,000 hours or more. The GaN semiconductor lasers are very prospective, and researches and developments have widely been conducted for GaN semiconductor lasers.
Thus, the GaN compound semiconductor is an excellent material which meets necessary conditions for light sources of the next-generation optical disk systems.
In the meantime, in order to fabricate a semiconductor laser, it is an indispensable condition that light and carriers are confined within an active layer. For this purpose, an AlGaN layer must be used as a cladding layer. In order to obtain a wavelength applicable to an optical disk system, etc. operable at about 400 nm, it is necessary that the Al composition (or composition ratio) in AlGaN be 25% or more and the thickness of each of symmetrical waveguides be 0.3 .mu.m or more.
However, the following problems will arise when the semiconductor laser having the AlGaN layer with high Al composition.
A tensile strain occurs due to a difference in lattice constant between the AlGaN layer and an adjacent GaN layer or a counter substrate. If the thickness of the AlGaN layer exceeds a critical film thickness, a hexagonal crack is made in the surface of the AlGaN layer due to the tensile strain. When a semiconductor laser comprises two kinds of semiconductor layers (i.e. main and sub-semiconductor layers), the critical film thickness refers to a critical film thickness of the sub-semiconductor film at which a crack or dislocation occurs. It is generally thought that a crack, etc. is made in the sub-semiconductor layer at the critical film thickness owing to a strain, etc. resulting from a difference in lattice constant between the main semiconductor layer and the sub-semiconductor layer. If the thickness of the sub-semiconductor layer is much less than its own critical film thickness, no crack, etc. is made. It should be noted that the critical film thickness varies, depending on the kind of semiconductor, combination of materials, and other conditions.
The difference in lattice constant between AlN and GaN is about 2%. Accordingly, about 0.5% of a difference in lattice constant is present between GaN and AlGa containing 20 to 30% of Al, and thus a strain occurs therebetween. When an Al-containing layer is simply grown on a sufficiently thick GaN underlayer, the GaN underlayer functions as a main layer and the growth of the Al-containing layer is influenced by the lattice constant of the GaN underlayer. A tensile strain acts in the AlGaN layer, and a crack will inevitably made if the thickness thereof exceeds the critical film thickness. Specifically, the thickness of an AlGaN cladding layer, which is required to confine light, is about 0.2 to 0.5 .mu.m. This thickness exceeds the critical film thickness of the AlGaN cladding layer in relation to the main layer, i.e. the GaN layer. In this case, a crack will form under normal conditions.
Once the crack has formed, it becomes difficult to flow a current in the semiconductor device in the direction of lamination. The resistance of the device exceeds 50 .OMEGA.. With this laser, laser oscillation itself is difficult. Even if the laser oscillates, the reliability of the device is very low. While power is supplied to the device, the characteristics are greatly degraded due to possible residual strain.
In the above example, gallium nitride, one of III-V compound semiconductor containing nitrogen, is mainly applied to the laser. Gallium nitride, GaN, however, is applied not only to lasers but also to various devices such as light-emitting elements, electronic devices and power devices.
The band gap of GaN is 3.4 eV and large, and GaN is a direct transition type compound. Accordingly, GaN is prospective as a material of a short-wavelength light-emitting element, as mentioned above. In addition, a GaN-based material obtained, e.g. by alloying GaN with indium nitride (InN), permits control of a wide band gap. Thus, much attention has been paid to the GaN-based material as a material of a light-emitting device for emitting orange light to ultraviolet light. Furthermore, much attention has been paid to the application of the GaN-based material to power devices, high-temperature operating devices, etc. by making use of its wide band gap.
A substrate for a GaN-based thin film material is required to be stable at high temperatures for growth of the GaN-based material and to have a small difference in lattice constant between the substrate itself and the GaN-based material. In a metal organic chemical vapor deposition (MOCVD) process, sapphire is widely used as a material of a device formation substrate since a substrate with a surface having relatively good characteristics can be obtained and wafers of sapphire with a diameter of about 2 inches can be easily obtained.
However, the degree of mismatching between sapphire and GaN is about 16% and consequently GaN tends to grow in an insular shape on a sapphire substrate. In addition, the dislocation density in a thin film of GaN is about 10.sup.10 cm.sup.-2 and high. When a sapphire substrate is used, e.g. in a light-emitting device, the light-emission efficiency is low, the operation voltage is high and the yield is low.
For example, a GaN-based light-emitting diode with a light emission wavelength of 520 nm, which is formed on a sapphire substrate, exhibits the following characteristics. As regards light emission efficiency, when an electric current of 20 mA is supplied, the external quantum efficiency is 6%. In this case, the operation voltage is 5V. As regards the life of the diode, when an electric current of 40 mA is supplied, the possibility of defects in a turn-on time period of 1000 hours is 25%. Thus, further increases in light emission efficiency and life and a decrease in operation voltage are to be desired.
In the formation of transistors, which serve as operational elements of power devices, high-temperature devices, high-speed operation devices, etc., there is a problem in a hetero-junction. It is still difficult to put such transistors to practical use.
As in the case of lasers, the problems with the above-described semiconductor devices result from cracks forming in or near Ga.sub.1-x Al.sub.x N layers.
Although the possibility of formation of cracks can be reduced by decreasing the value "x" in the composition Ga.sub.1-x Al.sub.x N, it is known that in this case the life of, in particular, a light-emitting diode is considerably decreased. For example, in the case of a laser diode, the possibility of formation of cracks can be reduced by decreasing the value "x" in the composition Ga.sub.1-x Al.sub.x N. However, the value of current density increases greatly during laser oscillation and continuous oscillation at room temperature is prevented. In the case of a high-speed operation element, a two-dimensional electron gas is not fully produced if the value "x" is decreased.
As described above, in the conventional GaN-based semiconductor device, it is very difficult to form an AlGaN layer with high Al composition as a cladding layer, etc. since the device resistance increases considerably. In other words, in the GaN-based semiconductor device, it is not possible to increase the life and enhance the operational characteristics at the same time. As a result, in the case of a laser, for example, it is difficult to achieve continuous oscillation.